When Io emerges from eclipse by Jupiter, the sodium D-line
emissions are weak, partly because of the small Doppler
shift of Io relative to the Sun, but also because the
neutral sodium density is a factor of two to four times
smaller than when Io has been out of the shadow and exposed
to sunshine for an hour or two. We have observed four cases
of post-eclipse brightening of the sodium emission, using a
10 arc second aperture image slicer to obtain images of
sodium around Io. These show that initially, most of the
sodium is confined to the “banana cloud” near Io. As time
passes, the sodium density close to Io increases, and after
and hour or two, most of the sodium is found surrounding Io.
We have thought of two possible explanations for the
post-eclipse sodium growth. One has to do with condensation
of sulfur dioxide during the eclipse, covering particles of
NaCl, which are thought to be the source of sodium (Hunten,
2003). As Io emerges from eclipse, the sulfur dioxide
sublimes, exposing the NaCl to ion sputtering. Another
possibility is that the sputtering process, which yields
sodium, is strongly temperature dependent, such that yields
are low during cold time during the eclipse, and then
increase rapidly as Io emerges into warming sunshine.
Laboratory experiments by Yakshinskiy et al. (2004) have
shown that sputtering yields of sodium are, in fact,
strongly temperature dependent. The NASA Planetary Astronomy
Program supported this work.